This application relates to speed limiting of internal combustion engines. In particular, the application relates to speed limiting of small internal combustion engines provided with a magneto ignition system.
On some small engine applications, there is a need to limit the maximum speed of the engine to some specific value, either for safety reasons or for limiting the speed of a vehicle on which such an engine is used. This may be necessary to prevent excessive mechanical stresses upon the engine itself, or to accessories attached to the engine, including the magneto flywheel and other such components. One particular application includes hand-held cutting tools powered by such a small engine, where a rotating cutting or drive element may burst when rotated at an excessive speed. In such applications it is common to operate such an engine with its throttle completely open while cutting, for maximum cutting speed and efficiency. However, when the cut is completed, or when the tool is lifted from the work, full-throttle operation may result in dangerous overspeeding.
Speed limiting to preventing such dangerous overspeeding is conventionally accomplished with a mechanical governor operating the throttle of the engine. Such an arrangement is subject to mechanical wear and is easily tampered with, defeating the speed limiting provision. Another known speed limiting method involves interrupting the spark voltage when the engine exceeds a predetermined speed, causing the engine to misfire, and slow down due to its internal friction. The instant invention provides a control circuit for interrupting the ignition of an engine rotating above a predetermined speed which utilizes a minimum number of components, is reliable and repeatable in operation, compact and light in weight, overcoming numerous deficiencies of the prior art.